In robots for industrial applications, a D.C. electromagnetic brake as illustrated in FIG. 1 has heretofore been employed for a mechanism which pulls up or lowers heavy articles.
The D.C. brake 3 shown in FIG. 1 consists of a brake housing 31 secured to a robot frame 1, a brake disc 32 fastened to a feed screw 2, an armature disc 33 disposed facing the brake disc 32, a spring 34 which works to press the armature disc 33 onto the brake disc 32, and a brake coil 35 which works to pull the armature disc 33 away from the brake disc 32. When the brake coil 35 is not energized, the armature disc 33 is pressed onto the brake disc 32 being urged by the spring 34, whereby the feed screw 2 is fastened to the robot frame 1 via brake 3. When the brake coil 35 is energized, on the ohter hand, a magnetic force is established between a yoke 36 provided around the brake coil 35 and the armature disc 33. The armature disc 33 is therefore pulled away from the brake disc 32, the braking function is released, and the feed screw is allowed to rotate freely.
The control circuit illustrated in FIG. 2 has hitherto been used to control the operation of the D.C. electromagnetic brake of FIG. 1. In the circuit of FIG. 2, a brake coil for the D.C. electromagnetic brake and a transistor 5' for controlling the current flowing through the brake coil are connected in series across the terminals of a D.C. power source 8'. The base of the transistor 5' which controls the current is connected to the output terminal of a transistor driver circuit 7'.
In the circuit of FIG. 2, a circuit 6' for restraining the counter electromotive force is connected in parallel with the brake coil in order to prevent the transistor 5' from being damaged by the counter electromotive force that generates across the terminals of the brake coil when the supply of current is interrupted. A circuit consisting of a resistor and a capacitor connected in series may be used as the circuit 6' for restraining the counter electromotive force. This, however, is not sufficient to absorb the counter electromotive force. Further, a diode may be used as the circuit 6' for restraining the counter electromotive force. The diode is capable of absorbing the counter electromotive force, but requires an extended period of time before the current flowing through the brake coil is completely interrupted, and causes the operation to become unstable. It has also been proposed to use a varistor having a suitable operation voltage to form the circuit 6' for restraining the counter electromotive force. FIG. 3 shows operation waveforms of the transistor which controls the flow of current, and FIG. 4 shows voltage vs. current characteristics of the varistor which is employed. As shown in FIG. 3, when the transistor 5' is conductive and the current is flowing into the brake coil, the voltage across the collector and emitter of the transistor 5' is nearly zero. When the transistor 5' is shifted from the conductive state to the nonconductive state in response to a signal from the transistor driver circuit 7', the voltage across the collector and emitter of the transistor 5' rises suddenly, reaches the power source voltage Vs, and then becomes greater than Vs due to the counter electromotive force generated across the coil. However, the counter electromotive force generated across the coil is limited by a conductive voltage Va of the varistor, and, hence, the voltage across the collector and the emitter of the transistor 5' is permitted to rise up to Vs+Va. The voltage across the collector and the emitter, which is clamped to the above-mentioned value, decreases with the decrease in the counter electromotive force of the coil and remains stable at the power source voltage Vs. Therefore, the operation time of the brake is reduced or, in other words, the braking operation is stabilized.
In the above-mentioned circuit, however, the conductive voltage Va of the varistor must be greater than the voltage which is applied to the brake coil and must, hence, be greater than the power source voltage Vs. Therefore, the transistor 5' must have a rated voltage which is two or more times as great as the power source voltage. However, transistors having such a high rated voltage are expensive and are not readily available.